1. Field of the Invention
The present invention relates to a high frequency amplifier used in cellular phones and S- and C-band wireless communications.
2. Background Art
In cellular phone and S- and C-band wireless communication networks, each coverage area is assigned a particular frequency, or frequency band, which can be used by communication devices. However, communication device manufacturers desire to avoid manufacture of a different high frequency amplifier for each such assigned frequency, since this complicates the management of the manufacturing process. Therefore, some high frequency amplifiers are designed to be operable in a plurality of frequency bands.
A known such high frequency amplifier will be described with reference to FIG. 13. In the high frequency amplifier shown in FIG. 13, the signal which has been amplified by an amplifying active device 12 is passed through a transmission line to the output terminal. Further in this high frequency amplifier, chip capacitors 21 and 51 are shunt-connected to the transmission line for matching purposes. A chip inductor and a diode 52 are connected to the chip capacitor 51. It should be noted that the diode 52 can be turned on and off by selectively controlling the application of the voltage from a terminal 53. That is, the chip capacitor 51 can be electrically connected to and disconnected from ground by turning on and off the diode 52. This allows the impedance of the high frequency amplifier to be set to two values to achieve the optimum matching conditions in two different frequency bands.
Thus, in some conventional high frequency amplifiers, a chip capacitor, etc. can be connected to and disconnected from ground by use of an active device such as a diode so that these amplifiers can be operated in a plurality of frequency bands. Further, high frequency amplifiers capable of amplifying signals in a plurality of frequency bands are described in Japanese Laid-Open Patent Publication Nos. 11-127040 (1999), 09-186533 (1997), 10-126173 (1998), and 2008-113202.
As described above, in order to be operated in a plurality of frequency bands, a high frequency amplifier must include an active device and a circuit for controlling it, resulting in an increased parts count. (The term “control circuit” is hereinafter used to refer collectively to the active device and the circuit for controlling it, which device is used to change the matching conditions of the high frequency amplifier.)
Further, such high frequency amplifiers are also disadvantageous in that powering the additional control circuit results in a decrease in the overall operating efficiency.